Most aren’t specifically for learners, but you could try:

!learntopaint@lemmy.world

!artshare@lemmy.world

!aip@lemmy.mindoki.com

!art@lemmy.world

!drawing@lemmy.world

!arting@lemmy.ml

!watercolor@lemmy.ml

These don’t have any activity, but are more aimed at learning:

!artistlounge@lemmy.ml

!learnart@lemm.ee

!artfundamentals@lemm.ee